1. Field of the Invention
The present invention relates to a method of producing a thin film of zinc oxide which is applied to a transparent conductive layer for a stacked structure forming a photovoltaic element such as a solar cell or the like, a process for manufacturing a photovoltaic element using its method, and the photovoltaic element, in particular, to a method of producing a thin film of zinc oxide by electrochemically depositing zinc oxide from an aqueous solution to form a thin film of zinc oxide.
2. Related Background Art
Formerly, to improve the effect of collecting longer wavelengths, a photovoltaic element made of hydrogenated amorphous silicon, hydrogenated amorphous silicon germanium, hydrogenated amorphous silicon carbide, microxe2x80x94or poly-crystalline silicon, or the like has a reflective layer provided on the back surface thereof. It is desirable that the reflective layer exhibits reflective characteristics effective in a range of 800 to 1200 nm, that is, wavelengths near the energy bandgap edge of a semiconductor material at which optical absorption is made smaller. A substance fully satisfying this condition includes metals such as gold, silver, copper and aluminum.
Besides, for the purpose of optical confinement, an uneven layer optically transparent in a predetermined wavelength range has been provided. The uneven layer has generally been provided between a reflective layer made of the above metal and a semiconductor active layer to effectively use a reflected light, thereby improving the short circuit current density Jsc.
Furthermore, to prevent the degradation of characteristics due to shunt pass, a thin film made of an optically transparent conductive material or a transparent conductive layer has been provided between a back surface reflective layer and a semiconductor layer.
These thin films are deposited by using a method such as vacuum evaporation method or sputtering method, whereby characteristics of solar cells improve in the short circuit current density Jsc by 1 mA/cm2 or more.
In xe2x80x9c29p-MF-2 Optical Confinement Effect in a-SiGe Solar Cells on Stainless Steel Substratesxe2x80x9d, the Extended Abstract of the 51st Autumn Meeting of the Applied Physical Society (1990 Autumn), p. 747, xe2x80x9cP-IA-15, a-SiC/a-Si/a-SiGe Multi-Bandgap Stacked Solar Cells With Bandgap Profilingxe2x80x9d by Sannomiya et al., Technical Digest of the International PVSEC-5, Kyoto, Japan, p. 381, 1990 and so on, for example, a reflective layer composed of silver atoms is examined about the reflectivity and the texture structure. It is concluded in these that an increase in short circuit current due to the optical confinement has been achieved by forming two deposited silver layers formed under different conditions in substrate temperature to form an effective unevenness and using the layers in combination with a zinc oxide layer to complete a reflective layer.
Transparent conductive layers used for these optical confinement layers are deposited and formed by vacuum evaporation process using resistor heating or electron beams, sputtering process, ion plating process, CVD process or the like. But, because of problems such as a high production cost of a target material or the like, a great depreciation expense of a vacuum apparatus and a low use efficiency of materials, use of these film forming techniques leads to an extremely high cost of a photovoltaic element (solar cell), thereby forming a large hindrance in industrial applications.
As countermeasures against the above problems, the present inventors have already proposed a technique for forming a thin film of zinc oxide by using a liquid deposition method, that is, an electrochemical deposition method (Japanese Patent Application Laid-Open No. 10-140373, U.S. Pat. No. 5,804,466, etc.), which can be applied as the reflective layer of a photovoltaic element (solar cell) and combined with a metal layer formed by sputtering and a transparent conductive layer.
According to this thin-film forming technique, neither an expensive vacuum apparatus nor an expensive target is required, thereby enabling the production cost of a zinc oxide film to be remarkably reduced. Besides, since the thin film can be deposited also on a larger-area substrate, this method is promising for photovoltaic element, such as solar cell, requiring a larger-area.
However, the thin-film forming technique using the above electrolytic deposition method (electrochemical deposition method) has the following problems.
(1) Only a smooth surface film is formed and there are no means for the provision of an uneven shape (texture structure) effective in optical confinement on a deposited film, for the shortening of a deposition time for the promotion of mass productivity and so on.
(2) Especially in cases of raising the current density or elevating the concentration of an aqueous solution, the abnormal growth in the shape of a needle, ball, dendrite or the like is likely to occur beyond the order of microns on a deposited film. When this thin film of zinc oxide is used as the constituent of a photovoltaic element, it is considered to allow the abnormal growth of the thin film to induce a shunt pass in a photovoltaic element.
(3) Variation is likely to occur in the size of zinc oxide crystal grains, so that there was a problem in uniformity when a larger area film is formed.
(4) The obtained thin film is inferior in adhesion to a substrate to a film formed by the vacuum evaporation process using resistor heating or electron beams, the sputtering process, the ion plating process, the CVD process or the like.
The present invention has been made in view of the above problems, and its object is to provide a method of producing a thin film of zinc oxide, which is capable of forming a thin film of texture structure effective in optical confinement for a short time and of preventing the abnormal growth of a deposited film by electrolytic deposition advantageous for lowering a production cost, which can provide a thin film of zinc oxide excellent in the uniformity and adhesion of a film surface, and which can be applied to the stacked structure of a photovoltaic element to enhance the photoelectric characteristics and raise the mass productivity; and a process for manufacturing a photovoltaic element by using the above method; and a photovoltaic element.
To achieve the above object, the method of the present invention is a method of producing a thin film of zinc oxide, which comprises immersing a counter electrode and a conductive substrate as a negative electrode in an aqueous solution containing nitrate ions and 0.05 mol/liter or more of zinc ions, and passing a current between the counter electrode and the conductive substrate to electrochemically deposit zinc oxide on the conductive substrate from the aqueous solution, thereby forming a thin film of zinc oxide, wherein a film forming rate of the thin film is varied at least one time midway during an electrolytic deposition reaction for forming the thin film.
Since there is a positive correlation between the film forming rate and the inclination of unevenness of a film surface, most of the required thickness can be initially formed at a large film forming rate and a film having a large inclination of a surface unevenness can be obtained by varying the film forming rate to be made minimum at a final stage. Thus, the formed thin film of zinc oxide has a texture structure highly effective in optical confinement.
Besides, by varying the film forming rate after the thin film of zinc oxide is deposited by 3000 xc3x85 or more, that is, when the thickness thereof is 3000 xc3x85 or more, it is preferable to form a texture structure highly effective in optical confinement, and a film having a large inclination is formed after varying the film forming rate, so that a texture structure having a large optical confinement effect is efficiently obtained in a short time.
Besides, there is a positive correlation between the current density and the film forming rate, it is preferable that the current density is varied to easily change the film forming rate.
Further, by adding saccharose or dextrine to an aqueous solution in addition to nitrate ions and zinc ions, the abnormal growth occurring in a zinc oxide layer can be remarkably controlled, and film formation can be easily carried out at high concentration, whereby a zinc oxide thin film of texture structure suitable for optical confinement can be formed. Accordingly, a highly functional (short circuit currentxe2x80x94and conversion efficiency-improved) photovoltaic element improved in yields and excellent in adhesion can be continuously supplied stably.
Furthermore, it is preferable to previously deposit a zinc oxide thin film on a conductive substrate. At present, no technique of directly depositing a zinc oxide thin film excellent in optical characteristics on an aluminum surface has been found in the electrolytic deposition process. This is because an aluminum surface is boehmite-denatured and significantly decreases in reflecting ability under action of a hot acidic solution and the boehmite-denatured surface becomes a folded structure, thereby bringing the growth of zinc oxide into the shape of tucks or powders. Thus, by previously depositing a zinc oxide thin film on an aluminum surface, a zinc oxide thin film excellent in optical characteristics can be deposited even though aluminum is used as the metal layer. Besides, with relatively ease, a zinc oxide thin film with less abnormal growth can be formed efficiently and uniformly.
By continuously forming a zinc oxide thin film in a roll-to-roll system, the zinc oxide thin film of an uneven structure excellent in optical confinement can be produced.